Redox biomolecules undergo reversible reduction and oxidation and efficiently transfer electrons to a natural electron acceptor. They generally contain an active site prosthetic group or cofactor that mediates and participates in an oxidation and reduction pathway. Most commonly, the prosthetic group is based on dinucleotides such as flavin adenine dinucleotide (FAD) and nicotinamide adenine dinucleotide (NAD). The redox biomolecules oxidize and accept electrons from a substrate and then transfer the electrons by means of a reversible oxidation and reduction of the prosthetic group to an electron acceptor.
Biosensors can provide sensitive, rapid and low cost assays for detection of analytes in a sample. A biosensor essentially comprises a redox biomolecule that recognizes a target analyte and a transducer that converts the recognition event into a measurable signal. In one sense, the biosensor operates by “interrupting” the flow of electrons to the natural electron acceptor. The flow is detected as a current or voltage by an electrical circuit containing an electrode in proximity to the biomolecule. The transfer of electrons between the active site prosthetic group of the redox biomolecule and an electrode surface is thus an important factor in the efficient operation of biosensors.
It is generally observed that efficiency of electron transfer from redox biomolecules to the electrode surface of a biosensor is less than the highly efficient reduction of the natural electron acceptor. Several groups have investigated the modification of redox biomolecules by covalent attachment of a redox mediator, but found the electron transfer rate constants to be far lower than those between enzymes and their natural electron acceptor.